1. Field of the Invention
The present invention generally relates to CMOS-type solid state imaging devices, and particularly relates to a CMOS-type solid state imaging device which has a stabilized black level that is used to prevent signal level fluctuation caused by temperature changes.
2. Description of the Related Art
Solid state imaging devices are provided with shielded pixels called an optical black, and use signals from these pixels as a reference black level when outputting signals from the light receiving section. This is aimed at preventing the signal levels of the light receiving section from fluctuating in response to temperature variation. If intense light illuminates the vicinity of the optical black in the light receiving section, however, electric charges spilling from the photo diodes flow into the pixels of the optical black section, thereby raising the black level thereof. In this case, the signal levels of the pixels using the boosted black level as a reference level end up being lower than the signal levels of the pixels using a normal black level as a reference level. This significantly degrades the quality of reproduced images.
FIGS. 1A and 1B are cross-sectional views of solid stage imaging devices having a structure that prevents electrical charge from flowing into the optical black section (Japanese Patent Laid-open Application No. 56-78179).
In the first related-art example shown in FIG. 1A, an N-type semiconductor layer 73 serving as a drain is formed in a P-type semiconductor layer 72 provided on an N-type semiconductor substrate 71. The second related-art example shown in FIG. 1B is directed to a case in which a P-type semiconductor substrate 75 is used. In this example, a P-type semiconductor substrate 75 and a P-type semiconductor layer 76 sandwich an N-type semiconductor layer 77 that receives a reverse bias. A drain 78 is formed in the P-type semiconductor layer 76.
The first related-art example and the second related-art example both have a structure that accumulates electrical charge in potential wells formed by electrodes when the electrical charge is generated through photoelectric conversion.
In the related-art configurations, an electric-charge accumulating section and a drain are formed in the same semiconductor layer, thereby preventing seepage of electrical charge from the light receiving section into the optical black with an aim of stabilizing the black level. The first related-art configuration, however, is effective only in a case where an N-type semiconductor substrate is adopted. If this configuration is applied to CMOS-type solid stage imaging devices that generally employ P-type substrates, no effect is observed with respect to electric charge that is generated deep in the substrate. Further, since the drain is separated from the N-type semiconductor substrate, an effect of capturing electric charge is not sufficient.
The second related-art configuration employing a P-type semiconductor substrate is provided with the N-type semiconductor layer having a reverse bias applied thereto. Since the drain is separated from the N-type semiconductor layer, an effect of capturing electric charge is not sufficient.
Accordingly, the present invention is aimed at providing a CMOS-type solid state imaging device that prevents electric charge from flowing into an optical black section, thereby stabilizing a black level.
It is a general object of the present invention to provide a CMOS-type solid state imaging device that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art.
Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a CMOS-type solid state imaging device particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a solid state imaging device, including a semiconductor substrate, a light receiving section which is provided in a surface of the semiconductor substrate, and includes an array of pixels that accumulate electric charge through photoelectric conversion of incoming light, a light shielding section which includes an array of pixels formed in the surface of the semiconductor substrate and shielded from light, and a drain which is situated between the light receiving section and the light shielding section, and is formed to a depth deeper in the semiconductor substrate than the light receiving section and the light shielding section.
In the CMOS-type solid state imaging device according to the present invention described above, the light receiving section and the optical black section are formed separately, and the drain is formed between the light receiving section and the optical black section so as to reach a depth deeper in the substrate than the light receiving section and the optical black section. Through the function of the drain, it is possible to prevent electric charge leaking from the light-receiving section from flowing into the optical black section when intense light illuminates the light-receiving section.
That is, although electric charge generated in a P-type well of the light receiving section is accumulated at photo diodes, electric charge leaks out when the photo diodes are saturated upon illumination by intense light. Even when this happens, the leaked charge will be captured by the drain. Moreover, since an N-type semiconductor layer of the drain is formed deeper into the substrate than the P-type well, electric charge generated at a depth deeper than the P-type well will be captured by the drain without flowing into the optical black section. Such prevention of inflow of electric charge is effective against the electric charge generated in the P-type well as well as against the electric charge generated at a depth deeper than the P-type well, thereby making it possible to obtain a stable black level even when intense light illuminates the light-receiving section.